Optical film with sharpened bandedge

ABSTRACT

The present invention provides reflective films and other optical bodies which exhibit sharp bandedges on one or both sides of the main reflection bands. The optical bodies comprise multilayer stacks M 1  and M 2 , each having first order reflections in a desired part of the spectrum and comprising optical repeating units R 1  and R 2 , respectively. At least one of the optical repeating units R 1  and R 2  varies monotonically in optical thickness along the thickness of the associated multilayer stack.

This is a continuation of application Ser. No. 09/006,085 filed Jan. 13,1998.

FIELD OF THE INVENTION

The present invention relates generally to multilayer optical bodies,and in particular to multilayer films exhibiting a sharpened reflectivebandedge.

BACKGROUND OF THE INVENTION

The use of multilayer reflective films comprising alternating layers oftwo or more polymers to reflect light is known and is described, forexample, in U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.), U.S. Pat. No.5,103,337 (Schrenk et al.), WO 96/19347, and WO 95/17303. The reflectionand transmission spectra of a particular multilayer film dependsprimarily on the optical thickness of the individual layers, which isdefined as the product of the actual thickness of a layer times itsrefractive index. Accordingly, films can be designed to reflectinfrared, visible or ultraviolet wavelengths λ_(M) of light by choice ofthe appropriate optical thickness of the layers in accordance with thefollowing formula:λ_(M)=(2/M)*D _(r)  (Formula I)wherein M is an integer representing the particular order of thereflected light and D_(r) is the optical thickness of an opticalrepeating unit (also called a multilayer stack) comprising two or morepolymeric layers. Accordingly, D_(r) is the sum of the opticalthicknesses of the individual polymer layers that make up the opticalrepeating unit. D_(r) is always one half lambda in thickness, wherelambda is the wavelength of the first order reflection peak. By varyingthe optical thickness of an optical repeating unit along the thicknessof the multilayer film, a multilayer film can be designed that reflectslight over a broad band of wavelengths. This band is commonly referredto as the reflection band or stop band.

It is desirable for a reflection band to have a sharp spectral edge atthe long wavelength (red) and/or short wavelength (blue) side. However,the reflective films known to the art that contain an optical repeatingunit of varying optical thickness typically have moderately slopedbandedges which cause reflections outside of the desired wavelengths ofinterest. For example, if a reflective film is designed to reflectinfrared light while being transparent over the visible spectrum, asloped edge on the blue side of the reflection band may encroach intothe visible region of the spectrum, thereby resulting in unwantedcoloring of the infrared reflective film body. Such coloring can beavoided by designing the infrared film such that the infrared reflectionband is moved further into the infrared region, but this results insubstantial transmission of infrared light near the visible region ofthe spectrum.

In other situations, it may be desirable to design a reflective film orother optical body that reflects light over a selected range in thevisible region of the spectrum, e.g., a reflective film that reflectsonly green light. In such a case, it may be desirable to have sharpedges at both the red and blue sides of the reflection band.

Many prior art reflective films comprising multilayer stacks also show anumber of small reflection peaks near the reflection band. Thisso-called “ringing” also may introduce unwanted reflections. It has beensuggested in the art that, for multilayer films that consist of anoptical repeating unit of constant optical thickness, such ringing mightbe suppressed by adding a number of optical repeating units having anoptical thickness of half that of the other optical repeating unitsresponsible for the reflection band. However, while this approach mayeliminate ringing, it does not improve bandedge sharpness and, in fact,may worsen it. Furthermore, this approach requires the presence ofstrippable skins on multilayer extruded films, since it permits onlythin layers of specific optical thickness on the surface.

There is thus a need in the art for a reflective film, and a method ofmaking the same, that exhibits a sharp bandedge on one or both sides ofthe main reflection band, and that avoids the presence of ringing andother undesirable reflections. These and other needs are met by thepresent invention, as hereinafter described.

SUMMARY OF THE INVENTION

The present invention provides reflective films and other optical bodieswhich exhibit sharp bandedges on one or both sides of the mainreflection bands. The optical bodies of the present invention comprisemultilayer stacks M₁ and M₂, each having first order reflections in adesired part of the spectrum and comprising optical repeating units R₁and R₂, respectively. The optical repeating units R₁ and R₂ eachcomprise at least a first polymeric layer and a second polymeric layer,said first and second polymeric layers having associated with them anindex of refraction n₁ and n₂, respectively, the difference between n₁and n₂ being at least 0.05. The optical repeating unit R₁ variessubstantially monotonically in optical thickness along the thickness ofsaid multilayer stack M₁, and the optical repeating unit R₂ is ofsubstantially constant optical thickness along the thickness of themultilayer stack M₂. The optical thickness of optical repeating unit R₁is either less than or equal to the minimum optical thickness of opticalrepeating unit R₁ along the thickness of multilayer stack M₁, or elsethe optical thickness of optical repeating unit R₂ is equal to orgreater than the maximum optical thickness of optical repeating unit R₁along the thickness of multilayer stack M₁, or said optical repeatingunit R2 varies substantially monotonically in optical thickness alongthe thickness of said multilayer stack M₂ opposite to said substantiallymonotonic optical thickness variation of optical repeating unit R₁ andthe minimum optical thickness of optical repeating unit R₂ along thethickness of multilayer stack M₂ is substantially equal to the minimumoptical thickness of optical repeating unit R₁ along the thickness ofmultilayer stack M₁ or the maximum optical thickness of opticalrepeating unit R₂ along the thickness of multilayer stack M₂ issubstantially equal to the maximum optical thickness of opticalrepeating unit R₁ along the thickness of multilayer stack M₁.

BRIEF DESCRIPTION OF THE DRAWINGS

This patent application file contains at least one drawing executed incolor copies of this patent or patent application with color drawing (s)will be provided by the office upon request and payment of the necessaryfee.

The present invention is described in detail by reference to thefollowing drawings, without, however, the intention to limit theinvention thereto:

FIGS. 1 a to 1 e show the optical thickness variation of the opticalrepeating units R₁ and R₂ in multilayer stacks M₁ and M₂ to obtainbandedge sharpening at the red or blue edge of the reflection band;

FIG. 2 shows the optical thickness variation of the optical repeatingunits R₁, R₂ and R₃ in multilayer films M₁, M₂ and M₃ to obtain bandedgesharpening at the blue and red edges of the reflection band;

FIG. 3 shows the optical thickness variation of the optical repeatingunits R₁, R₂ and R₃ in multilayer films M₁, M₂ and M₃ in which M₁ and M₃have continuously changing slopes;

FIGS. 4 a and 4 b are schematic diagrams of a multilayer film consistingof two alternating polymeric layers;

FIG. 5 is a 3-dimensional schematic diagram of an optical repeating unitconsisting of two alternating layers of polymeric materials;

FIG. 6 is a 3-dimensional schematic diagram of an optical repeating unitconsisting of polymeric layers A, B and C arranged in an ABCB pattern;

FIG. 7 a is a layer thickness gradient profile showing a combined layerthickness gradient of LTG1 and LTG2;

FIG. 7 b is a computed spectrum illustrating the short wavelengthbandedge for the reflectance band created by layer thickness gradientLTG1 and the effect of adding the reverse gradient LTG2;

FIG. 8 a is a layer thickness gradient profile of a stack design havinga reverse gradient with an f-ratio deviation;

FIG. 8 b is a computational spectrum illustrating the improvement inbandedge sharpness afforded by the combination of LTG1 and LTG3;

FIG. 9 a is a layer thickness gradient profile for the combined stacksLTG1 and LTG4;

FIG. 9 b is a computational spectrum illustrating the improvementobserved with the layer thickness gradient of FIG. 9 a compared to theLTG1 case;

FIG. 10 a is a the layer thickness gradient profile in which the lowindex layer is linear for the entire stack for LTG1 and LTG5, but thehigh index component undergoes a gradient reversal in the LTG5 section;

FIG. 10 b is a computational spectrum illustrating the improvement seenwith the gradient of FIG. 10 a vs. the LTG1 case;

FIG. 11 a is a layer thickness gradient profile for a simple band passfilter made by introducing a step discontinuity in the layer thicknessprofile of a broad band reflecting stack;

FIG. 11 b is the calculated spectrum for the gradient of FIG. 11 a;

FIG. 12 a is a layer thickness gradient profile made with two gradedlinear thickness distributions and an additional non-graded quarter wavestack,

FIG. 12 b is the calculated spectrum for the gradient of FIG. 12 a;

FIG. 13 a is a layer thickness gradient profile illustrating a curvedlayer thickness profile; and

FIG. 13 b is the calculated spectrum for the gradient of FIG. 13 a.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions and conventions are used throughout thedisclosure:

Desired part of the spectrum: any continuous range of wavelengthsbetween 400 nm and 2500 nm, also called desired reflection band.

Optical repeating unit (ORU): a stack of layers arranged in a particularorder, which arrangement is repeated across the thickness of amultilayer film; the stack of layers have a first order reflection at awavelength according to Formula (I) above.

Intrinsic bandwidth, or optical repeating unit (ORU) bandwidth: Thebandwidth that an infinite stack of ORU's of identical thickness wouldexhibit, which is readily calculated from the matrix elements of thecharacteristic matrix M as defined by Born and Wolf, “Principles ofOptics”, Edition 5, page 67. For a quarterwave stack of two materialswith an index differential less than about 0.3, it is given, to a goodapproximation, by the absolute value of the Fresnel reflectioncoefficient for that interface.

Stopband: A reflectance band is defined in general as a spectral band ofreflection bounded on either side by wavelength regions of lowreflection. With dielectric stacks, the absorption is typically lowenough to be ignored for many applications, and the definition is givenin terms of transmission. In those terms, a reflectance band or stopband is defined in general as a region of low transmission bounded onboth sides by regions of high transmission.

In one preferred embodiment, a single reflectance band or stop band forp-polarized light has a continuous spectrum between any two successivewavelengths at which the transmission is greater than 50 percent, andincluding such successive wavelengths as endpoints, and where theaverage transmission from one endpoint to the other is less than 20percent. Such preferred reflectance band or stop band is described inthe same way for unpolarized light and light of normal incidence. Fors-polarized light, however, the transmission values in the precedingdescription are calculated in a way that excludes the portion of lightreflected by an air interface with the stack or the stack's skin layersor coatings.

Bandwidth of stop band: For such a preferred embodiment as described inthe preceding paragraph, the bandwidth is defined to be the distance, innm, between the two wavelengths within the band which are nearest each50 percent transmission point, at which the transmission is 10 percent.In commonly used terms, the bandwidths are defined by the 10 percenttransmission points. The respective blue and red (i.e., short and longwavelength) bandedges are then taken to be the wavelength at the abovedefined 10% transmission points. The transmission of the preferred stopband is taken to be the average transmission between the 10 percenttransmission points; If a reflectance band does not have high enoughreflectivity to satisfy the definitions of bandwidth and bandedge slopefor the preferred embodiment, then the bandwidth may be taken simply tobe the full width at half maximum (FWHM) where the maximum is the peakreflectance value.

Bandedge slope of stop band: The slope of a band edge as described inthe preceding paragraph is taken from the 50 percent and 10 percenttransmission wavelength points, and is given in units of percenttransmission per nm.

Pass band: A pass band is defined: in general as a spectral transmittingband bounded by spectral regions of relatively low transmission. Withthe multilayer color shifting film, the passband is bounded byreflective stopbands. The width of the pass band is the Full Width atHalf Maximum Transmission (FWHM)value.

Bandedge slope of pass band: Band edge slopes are calculated from thetwo points on a given bandedge nearest the maximum transmission point,the transmission values of which are 50 and 10 percent of the maximumtransmission value. In one preferred embodiment, the passband has lowtransmission regions on both sides of the transmission peak withtransmission minima of 10 percent or less of the transmission value ofthe peak transmission point. For example, in this preferred embodiment,a pass band having: a 50 percent transmission maximum, would be boundedon both sides by reflectance bands having 5 percent or lowertransmission minima. More preferably, the transmission minima on bothsides of the passband are less than 5 percent of the peak transmissionvalue of the passband.

Edge filter: reflectance filter having only one bandedge within thewavelength range of interest.

Multilayer film: a film comprising an optical repeating unit designed toreflect light over a particular range of wavelengths. The multilayerfilm may contain additional layers between the optical repeating unitsand which additional layers may or may riot be repeated throughout themultilayer film.

Monotonically varying layer thickness of an optical repeating unit alonga multilayer film: the thickness of the optical repeating unit eithershows a consistent trend of decreasing or increasing along the thicknessof the multilayer film (e.g., the thickness of the optical repeatingunit does not show an increasing trend along part of the thickness ofthe multilayer film and a decreasing trend along another part of themultilayer film thickness). These trends are independent oflayer-to-layer thickness errors, which may have a statistical variancewith a 1-sigma value as large as 5% or more. In addition, a localvariation in the optical repeating unit may cause ripples in the layerthickness profile which is not strictly monotonic by the mathematicaldefinition; but the ripple should be relatively small compared to thethickness difference between first and last optical repeating unit.

Maximum optical thickness of an optical repeating unit: the maximum of astatistical curve fit to the actual layer distribution containing randomerrors in layer thickness.

Minimum optical thickness or an optical repeating unit: the minimum of astatistical curve fit to the actual layer distribution containing randomerrors in layer thickness.

In-plane axes: two mutually perpendicular axes that are in the plane ofthe reflective film. For the sake of convenience, they are denoted asthe x-axis and the y-axis.

Transverse axis: an axis that is perpendicular to the plane of thereflective film. For sake of convenience, this axis is denoted thez-axis.

An index of refraction along a particular axis is referred to as n_(i),wherein i indicates the particular axis, for example, n_(x), indicatesan index of refraction along the x-axis.

Negative birefringence: the index of refraction along the transverseaxis is less than or equal to the index of refraction along bothin-plane axes (n_(z)<n_(x) and n_(y))

Positive birefringence: the index of refraction along the transverseaxis is greater than the index of refraction along both in-plane axes(n_(z)>n_(x) and n_(y)).

Isotropic: the indices of refraction along x, y and z-axes aresubstantially equal (e.g., n_(x)=n_(y)=n_(z))

Infrared region: 700 nm to 2500 nm

Visible region: 400 nm to 700 nm

The f-ratio is defined as:$f_{k} = \frac{n_{k}*d_{k}}{\sum\limits_{m = 1}^{N}\quad{n_{m}d_{m}}}$wherein f_(k) is the optical thickness of polymeric layer k, 1 is thenumber of layers in the optical repeating unit, n_(k) is the refractiveindex of polymeric layer k, and d_(k) is the thickness of polymericlayer k. The optical thickness ratio of polymeric layer k along anoptical axis j is denoted as f_(jk) and is defined as above, but withreplacement of n_(k) with the refractive index of polymer material kalong axis j ( ).

Skin layer: a layer that is provided as an outermost layer typicallyhaving a thickness between 10% and 20% of the sum the physical thicknessof all optical repeating units.

DETAILED DESCRIPTION

The construction of multilayer films in accordance with the presentinvention can be used in a variety of ways to obtain bandedge sharpeningat the red or blue side of the band or on both sides.

Bandedge Sharpening—Blue Edge

To obtain bandedge sharpening in accordance with the present inventionat the blue edge of the reflection band, a multilayer stack M₁ having anoptical repeating unit R₁ is combined with a multilayer stack M₂ havingan optical repeating unit R₂. Both multilayer stacks are designed tohave a first order reflection band in a desired region of the spectrum,e.g., in the infrared region. It is possible to produce a film or otheroptical body having a first order reflection band in a particular regionof the spectrum by selecting polymeric materials with appropriateindices of refraction and by manipulating the physical thickness of eachof the individual polymeric layers of an optical repeating unit suchthat the optical thickness of the optical repeating unit appears at thedesired wavelength as predicted by Formula (I) above. By varying theoptical thickness of the optical repeating unit in the multilayer film,the desired reflection over a particular range in the spectrum can beobtained. Preferably, the optical repeating unit R₁ of multilayer stackM₁ is monotonically varied in optical thickness such that the desiredreflection band is obtained. However, it is also possible to use severalmultilayer stacks comprising different optical repeating units to covera desired reflection band.

The optical thickness of optical repeating unit R₁ preferably increasesmonotonically along the thickness of multilayer stack M₁. Multilayerstack M₂ may comprise an optical repeating unit R₂ that is substantiallyconstant in optical thickness or the optical thickness of opticalrepeating unit R₂ may decrease monotonically along the thickness ofmultilayer stack M₂. If the optical thickness of optical repeating unitR₂ is substantially constant, the optical thickness thereof should beapproximately equal to the minimum optical thickness of opticalrepeating unit R₁ along the thickness of multilayer stack M₁. Preferablyin this embodiment, the optical thickness of optical repeating unit R₂is substantially equal to the minimum optical thickness of opticalrepeating unit R₁.

FIG. 1 a depicts this embodiment and shows a plot of the opticalthickness of optical repeating units R₁ and R₂ versus the opticalrepeating unit number in a reflective film made in connection with thepresent invention. In FIG. 1 a, multilayer stack M₁ comprises opticalrepeating unit R₁ of increasing optical thickness, and multilayer stackM₂ comprises optical repeating unit R₂ of substantially constant opticalthickness. A reflective film designed in accordance with FIG. 1 a willhave a sharpened bandedge on the blue side of the reflection band.

FIG. 1 b depicts another embodiment of the present invention that alsoleads to sharpening of the reflection band on the blue side. As shown inFIG. 1 b, multilayer stack M₂ in this embodiment comprises an opticalrepeating unit R₂ that decreases monotonically in optical thicknessalong the thickness of multilayer stack M₂. The minimum opticalthickness of optical repeating unit R₂ in this embodiment is such thatit is substantially equal to the minimum optical thickness of opticalrepeating unit R₁ along multilayer stack M₁.

Bandedge Sharpening—Red Edge

To obtain bandedge sharpening in accordance with the present inventionat the red end of the reflection band, a multilayer stack M₁ having anoptical repeating unit R₁ is combined with a multilayer stack M₂ havingan optical repeating unit R₂. Both multilayer films are designed to havea first order reflection in a desired portion of the spectrum, e.g., areflection band in the green part of the visible spectrum.

The optical thickness of optical repeating unit R₁ preferably increasesmonotonically along the thickness of multilayer stack M₁. Multilayerstack M₂ may comprise an optical repeating unit R₂ that is substantiallyconstant in optical thickness, or else the optical thickness of opticalrepeating unit R₂ may decrease monotonically along the thickness ofmultilayer stack M₂. If the optical thickness of optical repeating unitR₂ is substantially constant, the optical thickness thereof should beequal to the maximum optical thickness of optical repeating unit R₁along the thickness of multilayer stack M₁. Preferably in thisembodiment, the optical thickness of optical repeating unit R₂ issubstantially equal to the maximum optical thickness of opticalrepeating unit R₁.

FIG. 1 c depicts this embodiment and shows a plot of the opticalthickness of optical repeating units R₁ and R₂ versus the opticalrepeating unit number in a reflective film body in connection with thepresent invention. In FIG. 1 c; multilayer stack M₁ comprises opticalrepeating unit R₁ of increasing optical thickness, and multilayer stackM₂ comprises optical repeating units R₂ of substantially constantoptical thickness. A reflective film body designed in accordance withFIG. 1 c will exhibit a sharpened bandedge at the red end of thereflection band.

FIG. 1 d depicts another embodiment of the present invention that alsoleads to sharpening of the reflection band on the red side. As shown inFIG. 1 d, multilayer stack M₂ now comprises an optical repeating unit R₂that decreases monotonically in optical thickness along the thickness ofmultilayer stack M₂. The maximum optical thickness of optical repeatingunit R₂ in this embodiment is such that it is substantially equal to themaximum optical thickness of optical repeating unit R₁ along multilayerstack M₁.

Bandedge Sharpening—Both Edges

To obtain bandedge sharpening at both ends of the reflection band, threemultilayer stacks M₁, M₂ and M₃ can be combined as in the embodimentshown in FIG. 2. There, multilayer stack M₁ comprises an opticalrepeating unit R₁ that monotonically increases along the thickness ofmultilayer stack M₁. At the end of the stack, where R₁ has the minimumoptical thickness, multilayer stack M₁ is combined with multilayer stackM₂ that comprises optical repeating unit R₂ having a constant opticalthickness. The optical thickness of R₂ is either substantially equal (asshown in FIG. 2) or is less than the minimum optical thickness ofoptical repeating unit R₁. As already described above for obtainingbandedge sharpening at the blue edge of the reflection band, opticalrepeating unit R₂ can also decrease monotonically along the thickness ofmultilayer stack M₂.

At the end of the stack where optical repeating unit R₁ has its maximumoptical thickness, there is combined a multilayer film M₃ comprising anoptical repeating unit R₃ that has a substantially constant opticalthickness. As shown in FIG. 2, the optical thickness of R₃ is equal tothe maximum optical thickness of optical repeating unit R₁. As alreadydescribed above for obtaining bandedge sharpening at the red end,optical repeating unit R₃ can also decrease monotonically along thethickness of multilayer film M₃.

In each of the above described embodiments, the multilayer stacks M₁ andM₂ and, optionally, M₃ have been described as being physically next toeach other in the reflective film. However, this is not a requirement.In particular, the multilayer stacks may be spaced away from each otherin the reflective film body by additional multilayer stacks and/oradditional layers such as, for example, a layer which improves theadherence between the multilayer stacks. For example, multilayer stackM₂ in FIG. 1 a could equally well be present at the other end ofmultilayer stack M₁ as shown in FIG. 1 e. Similarly, the positions ofmultilayer stacks M₂ and M₃ in FIG. 2 can be interchanged as well.However, the preferred spatial positions of the multilayer stacks M₁, M₂and, optionally, M₃ relative to each other is that they join togethersuch that adjacent layers are of approximately equal optical thicknessas illustrated in FIGS. 1 a-1 d and 2, with no intervening materiallayers or spaces.

Bandedge sharpening can be obtained even if the multilayer stacks M₁,M₂, and M₃ are not adjacent or in the order illustrated in FIG. 1 e. Thematerials and their indices may even be different in each of the threemultilayer stacks. However, the most efficient use of optical layerswill occur when repeat units of the same or similar optical thickness(multilayer stacks having overlapping reflection bands) are opticallycoupled to enhance constructive interference between those layers. Thisconstraint also provides a guideline for the range of useful thicknessesof repeat units R₁, R₂ and R₃ in multilayer stacks M₁, M₂ and M₃. Forexample, in FIG. 1 d, as the repeat units in multilayer stack M₂ getprogressively thinner to the right, with progressive deviation from thethickness of the maximum repeat unit of M₁, the optical coupling forconstructive interference is progressively weakened between those layersat the extremities. If the minimum thickness repeat unit of M₂ is ofoptical thickness d that is outside of the intrinsic bandwidth of themaximum thickness repeat unit in M₁, then that minimum thickness unitwill not contribute appreciably to bandedge sharpening on the red sideof the reflection band of multilayer M₁.

A reflective film or other optical body made in accordance with thepresent invention can be manufactured, for example, by multilayerco-extrusion as described in more detail below. Alternatively, themultilayer stacks forming the reflective films or other optical bodiesof the present invention may be manufactured separately from each other(e.g., as separate, free-standing films) and then laminated together toform the final reflective film.

Optical Stack Designs

Layer thickness distributions for extended reflection bands may take theform of a variety of exponentially or linearly increasing functionalforms. Such optical stacks create an extended reflection band ofpre-determined bandwidth and extinction. If the same functional form ismaintained from beginning to end (first to last layer), then the slopesof the bandedges may not be as steep as desired. To increase the slopeof either the left or right bandedge, the functional form of the layerthickness distribution may change near the end points of the primarystack distribution such that the slope of the layer thicknessdistribution approaches zero.

To further sharpen the bandedges, additional layers with zero oropposite sign slopes may be added. For example, combined multilayeroptical stacks M₁, M₂, and M₃ can be constructed as shown in FIG. 3 inwhich there are no discontinuities in the first derivative of the(statistically averaged) layer thickness profile. In FIG. 3, M₂ itselfhas a slight band sharpening profile in that the slope at the beginningand end of M₂ is equal to zero. Stacks M₁ and M₃ are designed such thatthey also have zero slopes where they join M₂. The slopes of both M₁ andM₃ change continuously until, at their endpoints, their slopes are equaland opposite to that of the main stack M₂. In FIG. 3, M₁ consists ofrepeat units 1 to 10, M₂ of units 10 to 90, and M₃ of units 90 to 105.M₂ itself consists of 3 regions: M₂₁, M₂₂, and M₂₃, similar to theprofile in FIG. 2. M ₂₁ consists of units 10 to 20, M₂₂ from 20 to 80,and M₂₃ from 80 to 90. M₂₂ is a linear thickness profile.

Furthermore, the combined distribution curve M₁+M₂+M₃ may be part of alarger optical stack and can be in an interior or on the exteriorposition of a larger stack. Thus, films and other optical bodies can bemade in accordance with the present invention whose total constructionscontain multiple reflecting bands created by multiple sets of layerthickness gradients, all with their respective bandedge sharpening layergroups.

Typically, the optical thickness variation of an optical repeating unitin accordance with the present invention can be obtained by varying thephysical thickness of the polymeric layers of the optical repeatingunit. The optical thickness of a repeat unit is selected according tothe wavelengths selected to be reflected. Any range of wavelengthsoutside of the intrinsic bandwidth of the optical repeating unit can beselected by addition of optical repeating units having the appropriaterange of optical thicknesses. According to one particular embodiment inconnection with the present invention, the physical thickness of allpolymeric layers constituting the optical repeating unit is varied atthe same rate. For example, all polymeric layers of the opticalrepeating unit may be varied in thickness according to the same linearfunction.

In an alternative embodiment of the present invention, the physicalthickness of the polymeric layers of the optical repeating unit may bevaried differently. This is particularly preferred where it is desirableto obtain an optical thickness variation of the optical repeating unitR₂ or R₃ of multilayer films M₂ and M₃, respectively. For example, theoptical thickness of an optical repeating unit consisting of twoalternating polymeric layers may be monotonically varied in accordancewith the present invention by keeping the physical thickness of one ofthe layers substantially constant while varying the physical thicknessof the other layer in accordance with, for example, a linear function.Alternatively, both layers can be varied in physical thickness but inaccordance with different functions, e.g., different linear functions ordifferent subtile power law functions.

Several preferred embodiments of the present invention are illustratedin Table I and in the examples which follow. Table I lists four separatelayer thickness gradients. Each gradient is comprised of repeatingquarter wave layers of a high index material (n=1.75) and a low indexpolymer (n=1.50). The starting thickness and the thickness increment foreach successive layer is provided. A computer modeling program was usedto investigate the effect of several combinations of gradients on thebandedge steepness of the primary reflectance band.

TABLE I LTG1 LTG2 LTG3 LTG4 LTG5 Total number of layers 170 30 30 30 30High index beginning 154.6 112.4 112.4 112.4 112.4 layer thickness (nm)High index layer −0.4965 0.726 0.726 0 0.726 thickness increment (nm)Low index beginning 183.3 133.3 133.3 133.3 133.3 layer thickness (nm)Low index layer −0.5882 0.8608 0 0 −0.5882 thickness increment (nm)

EXAMPLE 1 Reverse Gradient

An example of a reverse gradient is shown in FIG. 7 a. This figure showsthe combined layer thickness gradient of LTG1 and LTG2. In this case,the bandedge sharpening gradient, LTG2, consists of 30 layers ofalternating high and low index materials, both of which increase inthickness to maintain an f-ratio of 0.5 from the first to last layerpair.

Another example of a reverse layer gradient is shown in FIG. 7 b. Thisfigure shows the short wavelength bandedge for the reflectance bandcreated by layer thickness gradient LTG1 and the effect of adding thereverse gradient LTG2. The addition of LTG2 results in an increase tothe edge slope. The bandedge slope without the addition of LTG2 is 1.1percent/nm. When LTG2 is added, the slope increases to 1.9 percent/nm.The layer thickness profiles are shown in FIG. 7 a.

EXAMPLE 2 Reverse Gradient with f-ratio Deviation

An example of a stack design having a reverse gradient with an f-ratiodeviation is shown in FIG. 8 a. This figure shows a film stack design ofonly one material component with a reverse thickness gradient while theother has a zero gradient in the added band sharpening stack of LTG3.This combination of LTG1 and LTG3 also shows an improvement in bandedgesharpness over the LTG1 case as seen in FIG. 8 b below. The bandedgeslope with LTG3 added is 7.3 percent/nm.

EXAMPLE 3 Zero Gradient

This example demonstrates bandedge sharpening for the case of zerogradient stacks LTG4 for both materials. The stack design of thisexample also produces a much sharper bandedge than LTG1 alone. Thebandedge slope in this case is 3.6 percent/nm.

FIG. 9 a shows the layer thickness gradient for the combined stacks LTG1and LTG4. LTG4 has a zero thickness gradient for both materials, andmaintains a constant ratio of thickness between the high and low indexlayers. Again, as shown in FIG. 9 b,substantial improvement is seencompared to the LTG1 case, with a bandedge slope of 3.6 percent/nmcompared to the value of 1.1 percent per nm for LTG1.

EXAMPLE 4 Gradient Sign Change by Only one Component

In this case, the layer gradient for the low index layer is linear forthe entire stack for LTG1 and LTG5, but the high index componentundergoes a gradient reversal in the LTG5 section, as shown in FIG. 10 abelow. The resulting spectra are shown in FIG. 10 b, and a substantialimprovement is seen vs. the LTG1 case, with the bandedge slopeincreasing from 1.1 percent/nm to 3.6 percent/nm.

Band Pass Filters

The fabrication of narrow bandpass transmission filters, sometimesreferred to as notch filters, can be made by using two broad reflectionbands which cover most of the appropriate spectrum except for a verynarrow band between their adjacent bandedges. If the band pass filter isto be of both narrow band and high transmission, then nearly verticalbandedges are required. Typical design techniques of the prior art, inwhich individual layer thicknesses of each layer in the stack isassigned a unique value, may be impractical for polymeric stacksinvolving hundreds of layers. The edge sharpening techniques describedherein are particularly useful in this case.

One preferred embodiment involves the use of band sharpening stackshaving continuously varying gradients. The resulting band pass filtershave a higher transmission than filters made with linear (constantgradient) layer thickness distributions. The following computer modeledexamples illustrate this improvement.

FIG. 11 a. A simple band pass filter can be made by introducing a stepdiscontinuity in the layer thickness profile of a broad band reflectingstack, as illustrated in FIG. 11 a. The calculated spectrum of such anotch filter, made with the two simple linear thickness distributions ofFIG. 11 a, is shown in FIG. 11 b. Without the band sharpening techniquesdescribed above, the bandedge slopes are not high enough to make anarrow band notch filter. The bandedges slopes are about 1.2 percent/nmand 1.4 percent/nm for the blue and red edges, respectively. TheBandwidth is 54 nm and the peak transmission value is 62 percent.

A notch filter can be made with two graded linear thicknessdistributions and additional non-graded quarter wave stacks as shown inFIG. 12 a. The flat (zero gradient) sections 48 are useful forsharpening the respective bandedges of the adjacent reflecting bands.With the additional layers concentrated at the two thickness values oneither side of the notch wavelength, a much sharper transmission bandcan be made. The calculated spectrum for the illustrated stack is givenin FIG. 12 b. The steepness of the bandedges of the notch filterspectrum of FIG. 12 b will increase with the number of layers includedin the band sharpening feature of the stack, as illustrated in FIG. 12a. The bandedge slopes are about 9 percent/nm for both the blue and rededges. The Bandwidth is 13.8 nm and the peak transmission value is 55.9percent.

The curved layer thickness profile of FIG. 13 a was created to improveupon a deficiency of the stack design and spectrum of FIGS. 12 a and 12b. The side band ripples of the layer thickness profiles of FIG. 12 aoverlap and limit the transmission of a notch filter. Note that the peaktransmission of the notch band in FIG. 12 b is only about 50%. Byintroducing a curvature to the band sharpening stack thickness profile,the ringing at the edge of the spectrum of such a stack is reduced.Combining two such stacks will then make a notch filter with steeperbandedges and higher peak transmission, as illustrated by the resultsshown in FIG. 13 b. The bandedge slopes are about 12 percent/nm and 14percent/nm for the blue and red edges, respectively. The Bandwidth is 11nm and the peak transmission value is 76 percent. Note that, althoughthe bandwidth is narrower than in FIG. 12 b, the maximum transmission issignificantly higher. The number of layers in the band sharpeningportion of the stack is 60 on each side of the thickness gap, which isthe same number of layers used in the zero gradient sections of thelayer distribution of FIG. 12 a.

The curved profile can follow any number of functional forms. The mainpurpose of the form is to break the exact repetition of thicknesspresent in a quarter wave stack with layers tuned to only a singlewavelength. The particular function used here was an additive functionof a linear profile (the same as used on the remainder of thereflectance band) and a sinusoidal function to curve the profile withthe appropriate negative or positive second derivative. An importantfeature is that the second derivative of the layer thickness profile ispositive for the red bandedge of a reflectance stack and negative forthe blue bandedge of a reflectance stack Note that the opposite sign isrequired if one refers to the red and blue bandedges of a notch band.Other embodiments of the same principle include layer profiles that havemultiple points with a zero value of the first derivative. In all caseshere, the derivatives refer to those of a best fit curve fitted throughthe actual layer thickness profile which can contain small statisticalerrors of less than 10% one sigma standard deviation in layer thicknessvalues.

As illustrated by the above examples, the band sharpening profiles thatare added to the layer thickness distribution can have significanteffects on the slope of the bandedges, for one or both edges of areflectance band, and for the edges of a pass band. Sharp bandedges andhigh extinction are desirable in obtaining color filters havingsaturated colors of high purity. Preferably for reflectance bands, theslopes of the bandedges are at least about 1 percent per nm, morepreferably greater than about 2 percent per nm, and even more preferablygreater than about 4 percent per nm. The same slopes are preferred forbandpass filters having a bandwidth greater than or about 50 nm. Forpass band filters with a bandwidth of less than or about 50 nm, theedges are preferably greater than about 2 percent per nm, morepreferably greater than about 5 percent per nm, and even morepreferably, greater than about 10 percent per nm.

Design of the Optical Repeating Units

The polymeric layers of an optical repeating unit in accordance with thepresent invention can be isotropic or anisotropic. An isotropicpolymeric layer is a layer wherein the index of refraction of thepolymeric layer is the same independent of the direction in the layer,whereas in case of an anisotropic polymeric layer, the index ofrefraction will differ along at least two different directions. Thelatter type of polymeric layer is also called a birefringent layer. Todescribe an anisotropic polymeric layer, an orthogonal set of axes x, yand z is used as set out above in the definition section. Thus, ananisotropic polymeric layer will have at least two of the indices ofrefraction n_(x), n_(y) and n_(z) different from each other.

In one embodiment of the present invention, optical repeating units R₁,R₂ and/or R₃ consists of two alternating isotropic polymeric layers thathave an index of refraction differing from each other, preferably by atleast about 0.05 and more preferably by at least about 0.1. Morepreferably, however, at least one of the two alternating polymericlayers is a birefringent layer wherein at least one of the in-planeindices n_(x) and n_(y) differs by at least 0.05 from the correspondingin-plane index of refraction of the other layer. According to aparticular preferred embodiment in connection with the presentinvention, the index of refraction along the transverse axes (n_(z)) ofboth layers is substantially matched, i.e., the difference of the indexof refraction along the z-axes between both layers is preferably lessthan about 0.05. Optical repeating units of this type are particularlysuitable for reflecting light in the visible region of the spectrum, butmay also be used for reflecting light in the infrared region of thespectrum. Optical repeating units and multilayer films having thisfeature have been described in detail in WO 96/19347 and WO 95/17303,which are incorporated herein by reference. In another preferredembodiment of the present invention, the transverse index of the polymerlayer having the highest in-plane index is lower than the in-planeindices of the other polymer. This feature is also described in theabove-cited references.

FIGS. 4 a and 4 b illustrate these embodiments and show a multilayerfilm 10 comprising an optical repeating unit consisting of twoalternating polymeric layers 12 and 14. Preferably, at least one of thematerials has the property of stress induced birefringence, such thatthe index of refraction (n) of the material is affected by thestretching process.

FIG. 4 a shows an exemplary multilayer film before the stretchingprocess in which both materials have the same index of refraction. Lightray 13 experiences relatively little change in index of refraction andpasses through the film. In FIG. 4 b, the same film has been stretched,thus increasing the index of refraction of material 12 in the stretchdirection (or directions). The difference in refractive index at eachboundary between layers will cause part of ray 15 to be reflected. Bystretching the multilayer stack over a range of uniaxial to biaxialorientation, a film is created with a range of reflectivities fordifferently oriented plane-polarized incident light. The multilayer filmcan thus be made useful as reflective polarizers or mirrors. Ifstretched biaxially, the sheet can be stretched asymmetrically alongorthogonal in-plane axes or symmetrically along orthogonal in-plane axesto obtain desired polarizing and reflecting properties.

The optical properties and design considerations of multilayer stackscomprising two alternating polymeric layers is described most completelyin copending and commonly assigned U.S. Pat. No. 5,882,774, filed onMar. 10, 1995, the disclosure of which is hereby incorporated herein byreference. Very briefly, that application describes the construction ofmultilayer films (mirrors and polarizers) for which the Brewster angle(the angle at which reflectance goes to zero) is very large or isnonexistent for the polymer layer interfaces. This feature allows forthe construction of mirrors and polarizers whose reflectivity forp-polarized light decreases slowly with angle of incidence, isindependent of angle of incidence, or increases with angle of incidenceaway from normality. As a result, multilayer films having highreflectivity for both s- and p-polarized light over a wide bandwidth,and over a wide range of angles, can be achieved.

FIG. 5 shows an optical repeating unit 100 consisting of two polymericlayers, and indicates the three-dimensional indices of refraction foreach layer. The indices of refraction are n1x, n1y, and n1z for layer102, and n2x, n2y, and n2z for layer 104, respectively. Therelationships between the indices of refraction in each film layer toeach other and to those of the other layers in the film stack determinethe reflectance behavior of the multilayer stack at any angle ofincidence, from any azimuthal direction.

The principles and design considerations described in U.S. Pat. No.5,882,774 can be applied to create multilayer films having the desiredoptical effects for a wide variety of circumstances and applications.The indices of refraction of the layers in the multilayer stack can bemanipulated and tailored to produce devices having the desired opticalproperties. Many useful devices, such as mirrors and polarizers having awide range of performance characteristics, can be designed andfabricated using the principles described therein.

In accordance with another embodiment of the present invention, anoptical repeating unit of a multilayer film in accordance with thepresent invention comprises polymeric layers A, B and C having differentindices of refraction. Such type of repeating unit is particularlysuitable for designing an infrared reflective multilayer film. Inparticular, by selecting polymeric layers A, B and C such that polymericlayer B has an index of refraction intermediate that of polymeric layersA and C, an infrared reflective film can be designed for which at leasttwo successive higher order reflections are suppressed, thus allowingthe design of an infrared reflective film that is substantiallytransparent in the visible. A multilayer film of this type is describedin detail in, e.g., U.S. Pat. No. 5,103,337, which is also incorporatedherein by reference.

In accordance with this embodiment of the invention, multiplealternating substantially transparent polymeric layers A, B and C havingdifferent indices of refraction n are arranged in the order ABC.Additionally, the refractive index of polymeric layer B is intermediatethe respective refractive indices of the polymeric layers A and C. In aparticularly preferred embodiment having an optical repeating unitcomprising polymeric layers A, B and C arranged in a pattern ABCB, andwhere multiple successive higher order reflections are suppressed, theoptical thickness ratio f^(a) of first material A is ⅓, the opticalthickness ratio f^(b) of second material B is ⅙, the optical thicknessratio f^(c) of third material C is ⅓, and the index of refraction ofpolymeric layer B equals the square root of the product of the index ofrefraction of polymeric layers A and C. This particular type of opticalrepeating unit can be used to design a multilayer film in whichreflections for the second, third, and fourth order wavelengths will besuppressed.

In accordance with a further embodiment of the present invention, theabove multilayer film having an optical repeating unit comprisingpolymeric layers A, B and C arranged in an ABCB order can be designedusing an anisotropic layer for at least one of polymeric layers A, B andC. Thus, in accordance with one embodiment of the present invention, amultilayer film that reflects light in the infrared region of thespectrum while transmitting light in the visible region of the spectrummay comprise an optical repeating unit comprising polymeric layers A, Band C arranged in an ABCB order, the polymeric layer A having refractiveindices n_(x) ^(a) and n_(y) ^(a) along in-plane axes x and y,respectively, the polymeric layer B having refractive indices n_(x) ^(b)and n_(y) ^(b) along in-plane axes x and y, respectively, the polymericlayer C having refractive indices n_(x) ^(c) and n_(y) ^(c) alongin-plane axes x and y, respectively, polymeric layers A, B and C havinga refractive index n_(z) ^(a), n_(z) ^(b) and n_(z) ^(c), respectively,along a transverse axis z perpendicular to the in-plane axes, whereinn_(x) ^(b) is intermediate n_(x) ^(a) and n_(x) ^(c), with n_(x) ^(a)being larger than n_(x) ^(c) and/or n_(y) ^(b) is intermediate to n_(y)^(a) and n_(y) ^(c), with n_(y) ^(a) being larger than n_(y) ^(c) andwherein preferably at least one of the differences n_(z) ^(a)−n_(z) ^(b)and n_(z) ^(b)−n_(z) ^(c) is less than 0 or both said differences aresubstantially equal to 0.

By designing the optical repeating unit such that at least one of thedifferences n_(z) ^(a)−n_(z) ^(b) and n_(z) ^(b)−n_(z) ^(c) is less than0 and preferably less than −0.05, or such that both said differences aresubstantially 0, and while setting the index relationship along thein-plane axis between the layers as set out above, at least second andthird higher order reflections can be suppressed without a substantialdecrease of the infrared reflection with angle of incidence of theinfrared light.

The polymeric layers A, B and C of the optical repeating unit preferablyform an ABCB optical repeating unit. A schematic drawing of such arepeating unit is shown in FIG. 6. According to this embodiment, thedifference of the index of refraction between layers A and B along thez-axis (n_(z) ^(a)−n_(z) ^(b)) and/or the difference of the index ofrefraction between layers B and C along the z-axis (n_(z) ^(b)−n_(z)^(c)) is preferably negative, i.e., has a value less than 0, morepreferably less or equal to −0.05, and most preferably less than orequal to −0.1. It is particularly preferred to design the opticalrepeating unit such that: one of the differences is less than 0, morepreferably less than or equal to −0.05, and the other difference iseither equal to 0 or less than 0. Most preferably, both difference areless than 0. Such designs, wherein one of the difference is less than 0and the other is 0 or less than 0, yield an increase of the reflectionwith the angle of incidence.

It is also possible to design an optical repeating unit in accordancewith the present embodiment wherein both differences are substantially0, i.e., wherein the absolute value of the differences is preferablyless than 0.03. When both differences are substantially 0, there will belittle or no decrease in the infrared reflection with the angle ofincidence.

According to a still further species of the present embodiment, one ofthe differences in refraction index between layers A and B across thez-axis is of opposite in sign to the difference of the refraction indexbetween layers B and C across the z-axis. In the latter case, it ispreferred that the difference that is less than 0 has the largestabsolute value or that the absolute value of both differences issubstantially equal.

By adjusting the optical thickness ratios along the particular in-planeaxis that has the index of refraction for polymeric layer B intermediatethat of polymeric layer A and polymeric layer C, at least two higherorder reflections for infrared light having its plane of polarizationparallel to that particular in-plane axis can be suppressed. It is,however, preferred that the index of refraction for polymeric layer B isintermediate that of polymeric layers A and C along both in-plane axes,and by adjusting the optical thickness ratios along both in-plane axes,an infrared reflective mirror can be obtained for which at least twosuccessive higher order reflections are suppressed. Such an infraredreflective mirror will be substantially clear in the visible region andwill be free of color.

A particularly preferred optical repeating unit for designing aninfrared reflecting multilayer film in accordance with the presentinvention comprises polymeric layers A, B and C arranged in an ABCBpattern, with the refractive indices for polymeric layers A, B and Csuch that n_(x) ^(b)=(n_(x) ^(a)n_(x) ^(b))^(1/2) and/or n_(y)^(b)=(n_(y) ^(a)n_(y) ^(c))^(1/2) while keeping the following opticalthickness ratios: f_(x) ^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/orf_(y) ^(a)=⅓, f_(y) ^(b)=⅙ and f_(y) ^(c)=⅓. Such an embodiment iscapable of suppressing second, third and fourth order reflections. Aninfrared reflective multilayer film designed according to thisembodiment can be used to reflect infrared light up to about 2000 nmwithout introducing reflections in the visible part of the spectrum.

Preferably, an optical repeating unit comprising polymeric layers A, Band C has, along an in-plane axis, refractive indices of polymers A, Band C different by at least 0.05. Thus, it is preferred that n_(x) ^(a),n_(x) ^(b) and n_(x) ^(c) differ from each other by at least 0.05 and/orthat n_(y) ^(a), n_(y) ^(b) and n_(y) ^(c) differ from each other by atleast 0.05.

The above various embodiments describing different possible designs ofoptical repeating units for use in the multilayer films in accordancewith the present invention is not intended to be limiting to thisinvention. In particular, other designs of optical repeating units canbe used as well. Furthermore, multilayer films comprising opticalrepeating units of different design can be used in combination forforming a reflective film body in accordance with the present invention.For example, a multilayer film comprising an optical repeating unitconsisting of only two polymeric layers can be combined with amultilayer film comprising an optical repeating unit comprisingpolymeric layers A, B and C arranged in an ABC order in particular in anABCB pattern.

One skilled in the art will readily appreciate that a wide variety ofmaterials can be used to form (infrared) mirrors or polarizers accordingto the present invention when processed under conditions selected toyield the desired refractive index relationships. The desired refractiveindex relationships can be achieved in a variety of ways, includingstretching during or after film formation (e.g., in the case of organicpolymers), extruding (e.g., in the case of liquid crystallinematerials), or coating. In addition, it is preferred that the twomaterials have similar Theological properties (e.g., melt viscosities)so that they can be co-extruded.

In general, appropriate combinations may be achieved by selecting, foreach of the layers, a crystalline, semi-crystalline, or liquidcrystalline material, or amorphous polymer. It should be understoodthat, in the polymer art, it is generally recognized that polymers aretypically not entirely crystalline, and therefore in the context of thepresent invention, crystalline or semi-crystalline polymers refer tothose polymers that are not amorphous and includes any of thosematerials commonly referred to as crystalline, partially crystalline,semi-crystalline, etc.

Specific examples of suitable materials for use in the present inventioninclude polyethylene naphthalate (PEN) and isomers thereof (e.g.,2,6-,1,4-, 1,5-, 2,7-, and 2,3-PEN), polyalkylene terephthalates (e.,polyethylene terephthalate, polybutylene terephthalate, andpoly-1,4-cyclohexanedimethylene terephthalate), polyimides (e.g.,polyacrylic imides), polyetherimides, atactic polystyrene,polycarbonates, polymethacrylates (e.g., polyisobutyl methacrylate,polypropylmethacrylate, polyethylmethacrylate, andpolymethylmethacrylate), polyacrylates (e.g., polybutylacrylate andpolymethylacrylate), syndiotactic polystyrene (sPS), syndiotacticpoly-alpha-methyl styrene, syndiotactic polydichlorostyrene, copolymersand blends of any of these polystyrenes, cellulose derivatives (e.g.,ethyl cellulose, cellulose acetate, cellulose propionate, celluloseacetate butyrate, and cellulose nitrate), polyalkylene polymers (e.g.,polyethylene, polypropylene, polybutylene, polyisobutylene, andpoly(4-methyl)pentene), fluorinated polymers (e.g., perfluoroalkoxyresins, polytetrafluoroethylene, fluorinated ethylene-propylenecopolymers, polyvinylidene fluoride, and polychlorotrifluoroethylene),chlorinated polymers (e.g., polyvinylidene chloride andpolyvinylchloride), polysulfones, polyethersulfones, polyacrylonitrile,polyamides, silicone resins, epoxy resins, polyvinylacetate,polyether-amides, ionomeric resins, elastomers (e.g., polybutadiene,polyisoprene, and neoprene), and polyurethanes. Also suitable arecopolymers, e.g., copolymers of PEN (e.g., copolymers of 2,6-, 1,4-,1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylic acid, or esters thereof,with (a) terephthalic acid, or esters thereof; (b) isophthalic acid, oresters thereof; (c) phthalic acid, or esters thereof; (d) alkaneglycols; (e) cycloalkane glycols (e.g., cyclohexane dimethanol diol);(f) alkane dicarboxylic acids; and/or (g) cycloalkane dicarboxylic acids(e.g., cyclohexane dicarboxylic acid)), copolymers of polyalkyleneterephthalates (e.g., copolymers ofterephthalic acid, or esters thereofwith (a) naphthalene dicarboxylic acid, or esters thereof; (b)isophthalic acid, or esters thereof; (c) phthalic acid, or estersthereof; (d) alkane glycols; (e) cycloalkane glycols (e.g., cyclohexanedimethane diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkanedicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), and styrenecopolymers (e.g., styrene-butadiene copolymers and styrene-acrylonitrilecopolymers), 4,4′-bibenzoic acid and ethylene glycol. In addition, eachindividual layer may include blends of two or more of theabove-described polymers or copolymers (e.g., blends of SPS and atacticpolystyrene).

Particularly preferred birefringent polymeric layers for use in thepresent invention include layers containing a crystalline orsemi-crystalline polyethylenenaphthalate (PEN), inclusive of its isomers(e.g., 2,6-; 1,4-; 1,5-; 2,7; and 2,3-PEN). A particularly preferredisotropic polymeric layer for use in connection with this invention is alayer containing a polymethylmethacrylate, and in particular,polymethylmethacrylate itself.

It will further be understood by one skilled in the art that each of thepolymeric layers may be composed of blends of two or more polymericmaterials to obtain desired properties for a specific layer.

The films and other optical devices made in accordance with theinvention may also include one or more anti-reflective layers orcoatings, such as, for example, conventional vacuum coated dielectricmetal oxide or metal/metal oxide optical films, silica sol gel coatings,and coated or coextruded antireflective layers such as those derivedfrom low index fluoropolymers such as THV, an extrudable fluoropolymeravailable from 3M Company (St. Paul, Minn.). Such layers or coatings,which may or may not be polarization sensitive, serve to increasetransmission and to reduce reflective glare, and may be imparted to thefilms and optical devices of the present invention through appropriatesurface treatment, such as coating or sputter etching.

Both visible and near IR dyes and pigments are contemplated for use inthe films and other optical bodies of the present invention, andinclude, for example, optical brighteners such as dyes that absorb inthe UV and fluoresce in the visible region of the color spectrum. Otheradditional layers that may be added to alter the appearance of theoptical film include, for example, opacifying (black) layers, diffusinglayers, holographic images or holographic diffusers, and metal layers.Each of these may be applied directly to one or both surfaces of theoptical film, or may be a component of a second film or foilconstruction that is laminated to the optical film. Alternately, somecomponents such as opacifying or diffusing agents, or colored pigments,may be included in an adhesive layer which is used to laminate theoptical film to another surface.

It is preferred that the polymers have compatible rheologies forcoextrusion. That is, as a preferred method of forming the reflectivefilm bodies is the use of coextrusion techniques, the melt viscositiesof the polymers are preferably reasonably matched to prevent layerinstability or non-uniformity. The polymers used also preferably havesufficient interfacial adhesion so that the films will not delaminate.

The multilayer reflective film bodies of the present invention can bereadily manufactured in a cost effective way, and they can be formed andshaped into a variety of useful configurations after coextrusion.Multilayer reflective film bodies in accordance with the presentinvention are most advantageously prepared by employing a multilayeredcoextrusion device such as those described in U.S. Pat. Nos. 3,773,882and 3,884,606, the disclosures of which are incorporated herein byreference. Such devices provide a method for preparing multilayered,simultaneously extruded thermoplastic materials, each of which are of asubstantially uniform layer thickness.

Preferably, a series of layer multiplying means as are described in U.S.Pat. No. 3,759,647, the disclosure of which is incorporated herein byreference, may be employed. The feedblock of the coextrusion devicereceives streams of the diverse thermoplastic polymeric materials from asource such as a heat plastifying extruder. The streams of resinousmaterials are passed to a mechanical manipulating section within thefeed block. This section serves to rearrange the original streams into amultilayered stream having the number of layers desired in the finalbody. Optionally, this multilayered stream may be subsequently passedthrough a series of layer multiplying means in order to further increasethe number of layers in the final body.

The multilayered stream is then passed into an extrusion die which is soconstructed and arranged that stream-lined flow is maintained therein.Such an extrusion device is described in U.S. Pat. No. 3,557,265, thedisclosure of which is incorporated by reference herein. The resultantproduct is extruded to form a multilayered body in which each layer isgenerally parallel to the major surface of adjacent layers.

The configuration of the extrusion die can vary and can be such as toreduce the thickness and dimensions of each of the layers. The precisedegree of reduction in thickness of the layers delivered from themechanical orienting section, the configuration of the die, and theamount of mechanical working of the body after extrusion are all factorswhich affect the thickness of the individual layers in the final body.

The number of layers in the reflective film body can be selected toachieve the desired optical properties using the minimum number oflayers for reasons of film thickness, flexibility and economy. In thecase of both reflective polarizers and reflective mirrors, the number oflayers is preferably less than about 10,000, more preferably less thanabout 5,000, and (even more preferably) less than about 2,000.

The desired relationship between refractive indices of polymeric layersas desired in this invention can be achieved by selection of appropriateprocessing conditions used to prepare the reflective film body. In thecase of organic polymers which can be oriented by stretching, themultilayer films are generally prepared by co-extruding the individualpolymers to form a multilayer film (e.g., as set out above) and thenorienting the reflective film body by stretching at a selectedtemperature, optionally followed by heat-setting at a selectedtemperature. Alternatively, the extrusion and orientation steps may beperformed simultaneously. By the orientation, the desired extent ofbirefringence (negative or positive) is set in those polymeric layersthat comprise a polymer that can exhibit birefringence. Negativebirefringence is obtained with polymers that show a negative opticalstress coefficient, i.e., polymers for which the in-plane indices willdecrease with orientation, whereas positive birefringence is obtainedwith polymers having a positive optical stress coefficient. Thisterminology in the art of film orientation conflicts somewhat with thestandard optical definition of positive and negative birefringence. Inthe art of optics, a uniaxially positive birefringent film or layer isone in which the z-index of refraction is higher than the in-planeindex. A biaxially stretched polymer film such as PET will have highin-plane indices, e.g., 1.65, and a low out-of-plane or z-axis index of1.50. In the film making art, a material such as PET is said to bepositively birefringent because the index increases in the stretchdirection, but in the art of optics, the same material, after biaxiallystretching to film, is said to have uniaxial negative birefringencebecause the z-index is lower than the in-plane indices which aresubstantially equal. The term “positive birefringence” for a material asused herein will be that of the polymer film art, and will mean that theindex of refraction increases in the stretch direction. Similarly, theterm “negative birefringence” for a material will mean that the index ofrefraction of a film decreases in the direction of stretch. The terms“uniaxially positive” or “uniaxially negative”, when used in referenceto a birefringent layer, will be taken to have the meaning in the opticssense.

In the case of polarizers, the reflective film body is stretchedsubstantially in one direction (uniaxial orientation), while in the caseof mirrors the film can be stretched substantially in two directions(biaxial orientation). In the latter case, the stretching may beasymmetric to introduce specially desired features, but is preferablysymmetric.

The reflective film body may be allowed to dimensionally relax in thecross-stretch direction from the natural reduction: in cross-stretch(equal to the square root of the stretch ratio) or may be constrained(i.e., no substantial change in cross-stretch dimensions). Thereflective film body may be stretched in the machine direction, as witha length orienter, and/or in width using a tenter.

The pre-stretch temperature, stretch temperature, stretch rate, stretchratio, heat set temperature, heat set time, heat set relaxation, andcross-stretch relaxation are selected to yield a multilayer devicehaving the desired refractive index relationship. These variables areinter-dependent; thus, for example, a relatively low stretch rate couldbe used if coupled with, e.g., a relatively low stretch temperature. Itwill be apparent to one skilled in the art how to select the appropriatecombination of these variables to achieve the desired multilayer device.In general, however, a stretch ratio in the range from about 1:2 toabout 1:10 (more preferably about 1:3 to about 1:7) in the stretchdirection and from about 1:0.2 to about 1:10 (more preferably from about1:0.2 to about 1:7) orthogonal to the stretch direction is preferred.

Orientation of the extruded film can be accomplished by stretchingindividual sheets of the material in heated air. For economicalproduction, stretching may be accomplished on a continuous basis in astandard length orienter, tenter oven, or both. Economies of scale andline speeds of standard polymer film production may be achieved, therebyachieving manufacturing costs that are substantially lower than costsassociated with commercially available absorptive polarizers.

Two or more multilayer films may also be laminated together to obtain areflective film body in accordance with the present invention. Amorphouscopolyesters, such as VITEL Brand 3000 and 3300 from the Goodyear Tireand Rubber Co. of Akron, Ohio, are useful as laminating materials. Thechoice of laminating material is broad, with adhesion to the multilayerfilms, optical clarity and exclusion of air being the primary guidingprinciples.

It may be desirable to add to one or more of the layers, one or moreinorganic or organic adjuvants such as an antioxidant, extrusion aid,heat stabilizer, ultraviolet ray absorber, nucleator, surface projectionforming agent, and the like in normal quantities so long as the additiondoes not substantially interfere with the performance of the presentinvention.

The preceding description of the present invention is merelyillustrative, and is not intended to be limiting. Therefore, the scopeof the present invention should be construed solely by reference to theappended claims.

1. A reflective film comprising a plurality of optical repeating units,wherein the optical repeating units vary in optical thickness to form anoptical thickness profile that remains substantially constant over aplurality optical repeating units at a first optical thickness near onesurface of the film, and that monotonically varies from the firstoptical thickness to a second optical thickness through a centralportion of the film to define a reflection band.
 2. The reflective filmof claim 1, wherein the film is polymeric.
 3. The reflective film ofclaim 1, wherein the first optical thickness is lower than the secondoptical thickness.
 4. The reflective film of claim 1, wherein the firstoptical thickness is higher than the second optical thickness.
 5. Areflective film comprising a plurality of optical repeating units,wherein the optical repeating units vary in optical thickness to form anoverall optical thickness profile that remains substantially constantover a plurality of optical repeating units at a low optical thicknessnear one surface of the film, that remains substantially constant over aplurality of optical repeating units at a high optical thickness nearthe other surface or the film, and that monotonically increases from thelow optical thickness to the high optical thickness through theremaining central portion of the film to define a reflection band. 6.The reflective film of claim 5, wherein the film is polymeric.